1. The Field of the Invention
The invention relates to the field of optical devices. More particularly, this invention relates to the field of optical isolators for use in high speed optical networks.
2. The Relevant Technology
Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. This demand will continue to be fueled by high, bandwidth applications such as internet gaming and movie or music rentals and sales. This increased demand is occurring within and between metropolitan areas as well as within communications networks. These networks allow increased productivity and utilization of distributed computers or stations through the sharing of resources, the transfer of voice and data, and the processing of voice, data, and related information at the most efficient locations.
Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity. This increased demand, together with the growing number of distributed computing resources, has resulted in a rapid expansion of the number of fiber optic systems required.
Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates and high bandwidth capabilities. Other advantages of using light signals for data transmission include their resistance to electromagnetic radiation that interferes with electrical signals; fiber optic cables' ability to prevent light signals from escaping, as can occur with electrical signals in wire-based systems; and the ability of light signals to be transmitted over great distances without the signal loss typically associated with electrical signals on copper wire.
One optical device that allows for the effective communication of optical signals from a light emitting source, such as a laser, to an optical fiber is an optical isolator. Generally, an optical isolator is a component of an optical system that is used to block out reflected and unwanted light. One schematic configuration of an optical isolator is presented in FIG. 1, including a polarizer 102, a Faraday rotator 104 (formed of a magneto-optic (MO) crystal), a polarization analyzer 106 (“analyzer”), and a magnet 107. The components are depicted in duplicate in order to show the state of polarization of light in each of the forward and backward directions. Accordingly, each arrow in FIG. 1 indicates the state of polarization of the light passing through each of the polarizer 102, the Faraday rotator 104, and the analyzer 106.
The key to the function of an isolator is that the direction of rotation of the polarized light depends upon the direction of the magnetic field from magnet 107 and not on the direction of the light passing through the crystal. Thus, the MO crystal is not a reciprocal device, which makes almost unique among optical devices. Conceptually, the magnetic field tries to align the atoms of the MO crystal and, like pushing on a spinning top, causes them to precess. It is this precessional motion that causes the polarized light to rotate as it passes through the crystal. Since the precession direction only depends upon the direction of the magnetic field, the MO crystal is nonreciprocal from the point of view of the light. It is the thickness of the MO crystal that determines the degree of rotation of the light passing through.
Generally, a polarizer is an optical component that only passes light in a particular state of polarization, for example a vertical state of polarization. By arrow 108, it is indicated that light in a vertical state of polarization is passed through polarizer 102. In contrast, light in other states of polarization, such as horizontal, is not passed through the polarizer 102.
The Faraday rotator 104 receives light in the vertical state of polarization from polarizer 102. Generally, a Faraday rotator is an asymmetric device, which is usually an yttrium-iron-garnet (YIG) material, which rotates the state of polarization by a selected amount in one direction, for example clockwise, regardless of the direction of light propagation. The composition and crystal structure of an yttrium-iron-garnet (YIG) polarizer are well known in the art. The amount of rotation is determined by the thickness of the material. Thus, as indicated by arrow 110, light passing through Faraday rotator 104 is rotated 45° from vertical.
The light, now at a 45° angle from vertical, as indicated by arrow 112, then passes through analyzer 106. Generally, an analyzer is somewhat broader in function than a polarizer in that while a polarizer passes light in one state of polarization while absorbing other light, an analyzer separates light based on its state of polarization. However, in free-space isolators the analyzer and polarizer may be identical.
After light passes through the analyzer, some of the light is reflected back to the analyzer by other downstream optical devices. Of all the light reflected back to the analyzer, only the light with a 45° orientation of polarization is allowed to pass through the analyzer, as indicated by arrow 112. Thus, if the reflected light has a random polarization then the analyzer 106 attenuates the incoming reflection by 50% or 3 dB.
After passing through the analyzer, the reflected light is then rotated once more by Faraday rotator 104 in the clockwise direction, as indicated by arrow 114, to have a total rotation of 90°. However, light in this horizontal state of polarization cannot pass through polarizer 102 because it is rotated 90°, as indicated by short arrow 116. Thus, light can pass through optical isolator in a first direction through polarizer 102, Faraday rotator 104, and analyzer 106; but cannot pass in the opposite direction back through. More particularly, light that is polarized perpendicular to the polarizer is severely attenuated according to the extinction ratio of the polarizer, which is usually greater than 30dB. Hence, it is not that light cannot pass through the polarizer; it is that the light is attenuated by more than a factor of 1000. The effective result is that no light passes through the polarizer.
In addition, in order to minimize reflections of the light emitted from a laser from the polarizer surface back to the emitting laser, the polarizer surface must be tilted with respect to the laser propagation direction. Referring now to FIG. 2, in the conventional design the whole isolator chip assembly 200 and the magnet ring 202 are tilted by a few degrees (0-8 degrees), as shown in FIG. 2. Since the whole assembly 200 is tilted, the edge areas forming the tilt angles may block the transmission of the light and effectively reduce the aperture through which light can pass through the isolator, as indicated by dotted lines 204. Therefore, areas outside the aperture (above and below dotted lines 204) are not useful and they must be taken into account in the determination of the optical clear aperture requirements. Because of this, the isolator chip material is not as effectively used and a proportionally larger device must be designed.
In all cases, the material is by far the major factor of the isolator cost. One way to reduce the materials usage is to place isolators closer to the fiber connector. This has led to the design of ferrule mountable isolators that can be attached directly to the end surface of an APC connector with 8 degree tilt angle. In this case, the tilting of the polarizer surface is realized upon mounting to the APC fiber connector. This design has drastically reduced the requirement of clear aperture for the isolator chips and consequently, the overall cost of isolators.
While these tiny isolators are very useful for transmitter optical subassemblies (“TOSAs”) with angled APC connectors, they are not suitable for TOSAs with flat, LC Connectors because of the absence of tilt angle from the connector. This is a big disadvantage when the use of LC connectors is desired or required, as is the case with the Small Form-Factor Pluggable Transceiver MultiSource Agreement (“MSA”). Another drawback is that the standard APC connector has an 80 tilt angle, which is much bigger than necessary for minimizing polarizer surface reflection (typically 2-6 degrees). The bigger the tilt angle, the bigger the optical coupling loss. Therefore, these ferrule mountable isolators are not optimized for optical coupling purposes.